Method and apparatus for a non-oil -filled towed array with a novel hydrophone design and uniform buoyancy technique

ABSTRACT

An structure and method for constructing a non-oil-filled towed array providing a single cable entry point for each channel regardless of the number of hydrophones used to make up a hydrophone group or array. The method and apparatus enables uniform buoyancy of the hydrophone array and the primary cable around which the hydrophone array is wound. Uniform buoyancy is achieved through the selective addition of hollow micro-spheres into a Reaction Injection Molded (RIM) polyurethane material used to mold the hydrophones. For example, additional buoyancy may be desired adjacent heavier cable sections where connectors and telemetry modules are located. The preferred method and apparatus enables precise adjustment of hydrophone cable buoyancy by providing precise adjustment of the concentration of hollow glass microspheres in areas where more or less buoyancy is desired. Use of the preferred Reaction Injection Molding (RIM) method and apparatus of the present invention, results in significant reductions in cost and time in the construction of solid towed arrays. The RIM material enables use of a softer, lower durometer matrix in which the micro-spheres reside resulting in a hydrophone cable that is more flexible than prior known construction methods. For example, the prior known methods provide a minimum bending radius of 6 feet, whereas the method and apparatus of the present invention provide a minimum bending radius of 18 inches, which enables wrapping of the preferred hydrophone cable around a primary towing cable.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a system and method forhydrophones for sensing acoustic pressure waves. In particular theinvention relates to a technique for obtaining uniform buoyancy andincreased sensitivity utilizing a hollow micro sphere loaded reactioninjection molding polyurethane material.

[0003] 2. Description of the Related Art

[0004] Current methods of constructing non-oil-filled or solid towedarrays have proven to be prohibitively expensive and complex due totheir method of construction, which adversely affects reliability. Suchtechniques are described in U.S. Pat. Nos. 5,774,423; 5,361,240;5,883,857; and 4,733,378. As a result, solid towed arrays, whiledemonstrating substantial advantages over oil filled arrays, have metwith limited acceptance within the seismic and surveillance communitiesdue to their high price and unreliability. Thus there is a need for atechnology which addresses the primary causes of high cost and lowreliability with a unique method of construction that eliminates theprimary cause of failure and reduces the labor required to construct thearray.

[0005] The primary cause of failure of solid towed arrays results fromthe requirement to make electrical connections for each of the numeroushydrophones that make up a single channel in an array. In the case ofthe cable described in U.S. Pat. No. 5,883,857, the method ofconstruction calls for a discrete entry into the primary cable in orderto connect each hydrophone. Prior methods of construction have utilizeda “floatation” cable design that extrudes a buoyant material, such asfoamed polyethylene, over an inner jacketed cable, which is then coveredwith an outer extrusion of polyurethane. The primary problem with thisdesign is the fact that there is no bond between the polyethylene foamand both the inner and outer jackets since they are dissimilarmaterials. This procedure results in an effective path way between theunbonded dissimilar materials enabling water to migrate up and down thelength of the cable in the path way when the outer protective shield ofthe cable is damaged or a leak occurs relative to the point of cableentry where the hydrophones are attached. Attempted solutions to thisproblem have proven to be unreliable.

[0006] Thus, there is a need for a method of constructing a solid ornon-oil-filled hydrophone which does not enable formation of a path wayfor water to migrate up and down the length of the cable when the outerprotective cable shield is damaged or a leak otherwise occurs and whichprovides a cost effective method and apparatus for a solid hydrophone.

SUMMARY OF THE INVENTION

[0007] The above-mentioned long-felt need has been met in accordancewith the present invention which provides an apparatus for and method ofconstructing a non-oil-filled towed array. The present inventionprovides minimization of the cable entry points to one entry point foreach channel irregardless of the number of hydrophones used to make up ahydrophone group or array. The present invention also provides vastly.superior protection of the primary cable bundle from damage over presentknown methods.

[0008] The present invention provides for uniform buoyancy of thehydrophone array. Uniform buoyancy is achieved through the use of ahollow micro-sphere loaded Reaction Injection Molded (RIM) polyurethanematerial. Prior designs have relied on foaming in order incorporate airbubbles into molded areas in the hydrophone and hydrophone cable toachieve buoyancy. The result of prior methods, however, has been asignificant variation in the amount of buoyancy achieved. Prior methodshave been unable to precisely control the amount of buoyancy in areas ofthe cable where additional buoyancy is desired. For example, additionalbuoyancy is desired adjacent the approach to heavier sections of thehydrophone cable where connectors and in the case of digital arrays, thetelemetry modules are located.

[0009] This preferred method and apparatus of the present inventionenables precise adjustment of hydrophone cable buoyancy by providingprecise adjustment of the concentration of hollow glass micro-spheres inareas where more or less buoyancy is desired. Use of the preferredReaction Injection Molding (RIM) process of the present invention,results in significant reductions in cost and time in the constructionof solid towed arrays. The RIM material enables use of a softer, lowerdurometer matrix in which the micro-spheres reside resulting in ahydrophone cable that is more flexibility than prior known constructionmethods. For example, the prior known methods provide a minimum bendingradius of 6 feet, whereas the method and apparatus of the presentinvention provide a minimum bending radius of 18 inches.

[0010] The preferred method and apparatus of the present invention alsoprovides greater protection of the interior hydrophone cable due to thehomogeneous nature of the RIM material (which eliminates the necessityof layering of dissimilar materials and the creation of potential pathways for water to run up and down the hydrophone cable as discussedabove) which carries hydro phone signals and array telemetry, resultingin an improvement in the reliability of the system and greater immunityfrom damage due to abrasions or damage from other sources, for example,shark bite.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is an illustration of the flexible diaphragm and rigid backplane of the present invention;

[0012]FIG. 2 is a cross-sectional view of the hydrophone of the presentinvention showing the matching of the silver ink patterns with theconcave portions of the diaphragm;

[0013]FIG. 3 is a side view of the preferred hydrophone of the presentinvention prior to mounting the hydrophone on the cable;

[0014]FIGS. 4A and 4B illustrate a side view of the rigid tube andmolded plastic back plane of FIG. 2;

[0015]FIG. 5 a schematic representation of the piezo thin film elementwith pass through construction of the present invention;

[0016]FIG. 6 is an illustration of a cable having connector at both endsfor connection hydrophone sections together or connecting hydrophonesections to a transmission device such as a digital or analog telemetrymodel; and

[0017]FIG. 7 is a schematic representation of the molding of thehydrophone onto a hydrophone cable which is then helically wound onto aprimary cable.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0018] The preferred hydrophone provides a polymer film, air backedbender design, that unlike previous designs will not reduce thecircumferential length of the polymer film with an increase inhydrostatic pressure. The preferred method and apparatus of the presentinvention provides a polymer hydrophone that exhibits minimal change insensitivity with depth change and provides a substantially unlimitedcrush depth.

[0019] Turning now to FIG. 1, the preferred diaphragm 10 and back plane12 are illustrated. Diaphragm 10 slides over back plane 12. Minimalsensitivity to depth change and substantially unlimited crush depth areprovided by the preferred design comprising the hydrophone diaphragm 10having a series of concave faces 11 that run longitudinally along thelength of the hydrophone diaphragm 10 and discretely around thecircumference of the hydrophone diaphragm 12. In the example of FIG. 1,the eight concave surfaces of diaphragm 10 occupy the eight octantsaround the circumference of back plane 12. Each of the concave surfaces11 forms a discrete hydrophone bender element using one section of acontinuous piece of piezo film 20, shown in FIG. 5.

[0020] The back plane 12 of the hydrophone is molded onto a rigidtubular member shown in FIG. 2, in such a manner as to create acylindrical surface 15 at each end of the back plane. The back plan isprovided with longitudinal ridges 13 residing upon the rigid tubularmember 21, shown in FIG. 2. The molded back plane fits into the shape ofthe diaphragm 10 to form a surface against which the flexible diaphragm10 stops when sufficient hydrostatic pressure is applied. Stoppingagainst the back plane enable stretching under hydrostatic pressurewithout stretching the film beyond its yield point. Mounting of theflexible diaphragm 10 onto back plane 12 is illustrated in FIG. 2.

[0021] Turning now to FIG. 2, a cross section of the preferredhydrophone is illustrated. As shown in FIG. 2, diaphragm 10 slides overback plane 12 and back plane ridges 13. Air voids 14 are formed betweendiaphragm 10 and back plane cylindrical portion 15 and back plane ridges13. Hydrophone cable orifice 18 is surrounded by rigid tube 20. Rigidback plane 12, shown in FIG. 1, resides on top of rigid tube 21. Theentire structure is surrounded with molding 16. FIGS. 4A and 4Billustrate a side view of rigid tube 21 and molded back plane 12 havingcylindrical portion 15 and ridges 13.

[0022] Turning now to FIG. 5, a thin film homo-polymer piezo element 20is constructed to enable wires to be connected to two sides of the film20 with a flexible conductive ink 22 applied to both sides of thepolymer to form the piezo element 20. The front side of piezo element 20is visible in FIG. 5. The back side of piezo element 20 lookssubstantially similar to the front side and is not shown. The front sideof piezo element 20 is coated with a conductive silver ink pattern 22,which is connected to outside pins 24. The back side of piezo element 20is coated with a similar silver ink pattern and is connected to insidepins 26. This construction technique enables multiple piezo elements tobe connected together in either a series or parallel configurationwithout the necessity of wires to pass the signal across the piezoelement 20. The silver ink pattern sections 22 are interspersed withvoids 28. Connectivity across voids 28 between adjacent silver inksections 22 on the front side of piezo element 20 is provided bymetallic traces 30. Similarly, connectivity across voids 28 betweenadjacent silver ink sections 22 on the back side of piezo element 20 isprovided by metallic traces 30.

[0023] As shown in FIG. 2, voids 28 are positioned on top of and alignedwith back plane ridges 13 and silver ink portions 22 coincide withdiaphragm concave portions 11. This alignment provides for deformationof concave diaphragm portions 11 and silver ink sections 22 betweenridges 13. The arrangement of FIG. 2 reduces the effect of passivecapacitance contributions from ridge supported voids areas 28 of thepiezo element that are not deformed by incident vibration. The piezoelement 20 is bonded to the concave diaphragm 10 with a transferadhesive such as 3M-VHB using a press with a face having the same shapeas concave surface 11 of diaphragm 10 upon which the material 20 isadhered.

[0024] As shown in FIG. 2, back plane 12 provides ridges 13 runninglongitudinally down the length of the hydrophone, forming a series ofstandoffs that fit into the high points 17 of the diaphragm formed atthe joining edges of or space between adjacent concave surfaces 11. Thisconstruction serves to lock the high points 17 in place on back planeridges 13 and results in substantially all deformation of the film dueto dynamic pressure variation (sound) occurring between these fixedpoints at high point 17 which is coincident with void 28. The cumulativechange in length of the piezo film in the preferred embodiment isgreater than that of a simple cylinder resulting in improved sensitivityof the hydrophone for a given signal pressure level.

[0025] There are two methods of embodying the hydrophone for mounting onthe cable. As shown in FIG. 7, the outer covering for the hydrophoneconsists of a cylindrical sleeve 16, one end of which has an end that isof the same diameter as the back plane and another end that is of alarger diameter to allow for the insertion of the backplane/diaphragm/film assembly. Once the back plane/diaphragm/filmassembly is inserted and wire connections made between pins 24, 26 andtwisted pair 38, the space between the back plane/diaphragm/filmassembly and the inner wall of the outer sleeve 16 is filled with asuitable potting material. The hydrophone assembly 40 now has a singletwisted pair of wires 38 entering one end of the hydrophone assembly andexiting the other end of the hydrophone assembly. Back plane 12 ismolded from a material that is bondably compatible with the materialthat is used to form the outer covering of the hydrophone.

[0026] Cylindrical portion 15 of the back plane 12 extends beyond thelength of the sleeve to enable injection molding of end caps 14 on thehydrophone, sealing the hydrophone and the cable entry points. In thecase of this example the molding step also is used to form twooppositely located “cups” 42, as shown in FIG. 3 on each end of thehydrophone. The cups have an outer diameter equal to the maximumdiameter of the outer sleeve 16 formed on the ends of hydrophonesegments 40.

[0027] As shown in FIG. 7, after locating hydrophone assemblies 40 inthe desired locations along a primary cable 50, buoyant material 52 isinjected along the length of the cable between the hydrophones andinside the cups to which the material bonds. The resulting assemblyforms a constant diameter non-oil-filled towed array of significantlyreduced diameter. The buoyancy of the hydrophone cable and the primarycable around which the hydrophone cable is helically wrapped, can beselectively controlled to increase or decrease the desired buoyancy at adesired point along the hydrophone cable length, within the hydrophonesor within the hydrophone cups by increasing or decreasing the hollowmicrosphere concentration accordingly. In a preferred embodiment usingRIM, a folia material is used in molding the hydrophone cups and forms abondable interface between the RIM materials used in the forming thehydrophone cups and the overmolding of the hydrophone cable.

[0028] An alternative method of the present invention provides for useof RIM material, without hollow microspheres, to produce the moldedhydrophone structure described above and indicated in the FIGS. 1-7. Useof RIM material produces a lower cost hydrophone embodiment compared tothe first method and is accomplished with a single injection of RIMmaterial.

[0029] As shown in FIG. 7, in a preferred embodiment hydrophones areconstructed along a single strain relieved twisted pair cable 50 ofrelatively small diameter. Each hydrophone 40 is molded to enablelongitudinal pass-through of the cable 50 to attached subsequenthydrophones within a group. The length of the cable 50 betweenhydrophones is sufficient to enable winding the cable 50 helicallyaround a primary cable bundle 60, providing strain relief duringdeformation of the cable while towing. A single entry point into theprimary cable is made at the head of the hydrophone group whereappropriate electrical connections are made between signal conductors inthe primary cable and the twisted pair from the hydrophone group. Theconnections are made using standard take-out methods.

[0030] Substantially uniform neutral buoyancy of the entire primarycable section is accomplished with a syntactic Reaction Injection Moldedmaterial that provides high flexibility and consistent buoyancy thatwill not be reduced by over-pressurization. Control of the hollow sphereconcentration during Reaction Injection Molding of the hydrophone,associated hydrophone cable and primary cable is provided by a “ModeQuad Sure Shot 150” available from Hi Tech Engineering, Grand Rapids,Mich.

[0031] As shown in FIG. 6, in a preferred embodiment, a connector 70 canbe attached on the end of each hydrophone section 58 to enableconnecting the hydrophone sections together or connecting a hydrophonesection or sections to a transmission device such as a digital or analogtelemetry module. Additional micro spheres are added to the cable toincrease bouyancy at each end of the hydrophone section to compensatefor the additional weight of the connectors at the ends, therebymaintaining neutral buoyancy along the entire length of the section.

[0032] The foregoing description is intended as an example of apreferred embodiment and not intended to limit the scope of theinvention, which is defined by the following claims.

What is claimed is:
 1. A solid towed bender hydrophone comprising: adiaphragm having a tubular shape; a thin film piezo element attached tothe diaphragm; and a back plane having a cylindrical shape, furthercomprising at least one longitudinal rib on the exterior surface of theback plane wherein the back plane and the at least one rib slide intothe center of and slidably engage the tubular diaphragm.
 2. Theapparatus of claim 1 wherein the diaphragm further comprises a pluralityof discrete concave surfaces running longitudinally along the diaphragmwherein the at least one rib acts as an offset between the back planeand the diaphragm, each concave surface using a section of the piezofilm to form a discrete hydrophone bender element.
 3. The apparatus ofclaim 2 wherein the back plane and at least one rib are molded onto arigid tubular member.
 4. The apparatus of claim 2 wherein the back planehas a tubular shape having a hollow center through which a hydrophonecable passes.
 5. The apparatus of claim 1 wherein the continuous filmpiezo element has a front and back side and is constructed so that awire can be connected each side of the film with a flexible conductiveink.
 6. The apparatus of claim 2 wherein the thin film piezo elementfurther comprises a conductive ink pattern on each side of the film,wherein the pattern comprises a plurality of ink sections interspersedwith voids, wherein ink patterns adjacent a void are connected with aconductive strip across a void between adjacent ink patterns.
 7. Theapparatus of claim 6 further comprising a set of pins attached to thefront ink patterns and a set of pins attached to the back ink patternswhich enable connecting a plurality of hydrophones in series orparallel.
 8. The apparatus of claim 6 wherein the voids are positionedcoincident with the back plane ribs.
 9. The apparatus of claim 8 furthercomprising a reaction injection molding covering.
 10. The apparatus ofclaim 9 further comprising a molding cup formed on each end of theapparatus for receiving a hydrophone cable.
 11. The apparatus of claim10 further comprising a plurality of hydrophones physically attached endto end and wound helically around a primary cable.
 12. The apparatus ofclaim 11 wherein the plurality of hydrophones provide a singleattachment point to the primary cable.
 13. A buoyant hydro phone streamhaving a desired buoyancy along the stream comprising: a plurality ofhydro phones are connected together end to end on a hydro phone cable;and a hollow micro sphere loaded molding adjustably placed on the hydrophone cable between the hydro phones, wherein a density of micro spheresis adjusted to provide a desired buoyancy along the stream, so that afirst desired concentration of micro spheres are concentrated at heaviercable sections and a second desired concentration of micro spheres areconcentrated at lighter cable sections, to achieve the desired buoyancy.14. The apparatus of claim 13 wherein the micro spheres are combined ina soft reaction injection molding material to provide an eighteen inchminimum bending radius.
 15. A method for making a solid towed benderhydrophone comprising the steps of: providing a diaphragm having atubular shape; providing a thin film piezo element attached to thediaphragm; and providing a back plane having a cylindrical shape,further comprising at least one longitudinal rib on the exterior surfaceof the back plane wherein the back plane and the at least one rib slideinto the center of and slidably engage the tubular diaphragm.
 16. Themethod of claim 15 wherein the diaphragm further comprises a pluralityof discrete concave surfaces running longitudinally along the diaphragmwherein the at least one rib acts as an offset between the back planeand the diaphragm, each concave surface using a section of the piezofilm to form a discrete hydrophone bender element.
 17. The apparatus ofclaim 16 further comprising the step of molding the back plane and atleast one rib onto a rigid tubular member.
 18. The apparatus of claim 16wherein the back plane has a tubular shape having a hollow centerthrough which a hydrophone cable passes.
 19. The apparatus of claim 15further comprising the step of: connecting a wire to a back side of thefilm with a flexible conductive ink and connecting a wire to the frontside of the film with a flexible conductive ink.
 20. The method of claim16 further comprising the step of: placing a a conductive ink pattern oneach side of the film, wherein the pattern comprises a plurality of inksections interspersed with voids; and connecting the ink patternsadjacent a void with a conductive strip across a void between adjacentink patterns.
 21. The method of claim 20 further comprising step of:connecting a set of pins to the front side ink patterns; connecting aset of pins to the back side ink patterns, thereby enabling connectionof a plurality of hydrophones in series or parallel.
 22. The apparatusof claim 20 further comprising the step of: positioning the voids arecoincident with the back plane ribs.
 23. The apparatus of claim 22further the steps of: covering the apparatus with a reaction injectionmolding material.
 24. The method of claim 23 further comprising the stepof: forming a molded cup on each end of the apparatus for receiving ahydrophone cable.
 25. The apparatus of claim 24 further comprising thestep of: winding a plurality of hydrophones physically attached end toend and helically around a primary cable.
 26. The method of claim 25further comprising the step of: attaching the plurality of hydrophonesto the primary cable at a single attachment point.
 27. A buoyant hydrophone stream having a desired buoyancy along the stream comprising: aplurality of hydro phones are connected together end to end on a hydrophone cable; and a hollow micro sphere loaded molding adjustably placedon the hydro phone cable between the hydro phones, wherein a density ofmicro spheres is adjusted to provide a desired buoyancy along thestream, so that a first desired concentration of micro spheres areconcentrated at heavier cable sections and a second desiredconcentration of micro spheres are concentrated at lighter cablesections, to achieve the desired buoyancy.
 28. The apparatus of claim 27wherein the micro spheres are combined in a soft reaction injectionmolding material to provide an eighteen inch minimum bending radius. 29.A method for building a buoyant hydro phone stream having a desiredbuoyancy along the stream comprising the steps of: connecting aplurality of hydro phones are end to end on a hydro phone cable; moldinga hollow micro sphere loaded molding material onto the hydro phone cablebetween the hydro phones; adjusting a density of micro spheres toprovide a desired buoyancy along the hydrophone cable, according to theweight of the cable and attachments to achieve the desired buoyancy. 30.The method of claim 29 further comprising the step of: combining themicro spheres in a soft reaction injection molding material to providean eighteen inch minimum bending radius for the hydrophone cable.